Bacteria were responsible for the magnitude of the world-class hydrothermal base metal sulfide orebody at Navan Ireland

We report sulfur and lead isotope analyses of mine concentrates from Navan, the largest zinc deposit in Ireland. These samples, each representing up t o a million tonnes of ore, show very limited isotopic variation, with mean delta S-34 = -13.6 +/- 2 per mil (n = 20), and galena concentrate mean Pb-2 06/Pb-264 = 18.19 +/- 0.03 (n = 7). Calculations suggest greater than or eq ual to 90 percent of the Navan sulfides were derived through bacteriogenic reduction of Mississipian seawater sulfate, whereas metals were acquired fr om a local, orogenic crustal source beneath the ore-body. Enhanced bacteria l activity was fundamental to ore deposition at Navan: no bacteria, no gian t ore deposit.